The present invention relates to the packaging of integrated circuits (ICs) and more particularly to a method of attaching a capacitor to a substrate.
Solder paste, a specially blended paste of highly graded solder alloy powder particles in a flux medium, is commonly used for attaching a capacitor to a substrate. FIG. 1 illustrates a conventional method 10 of forming a semiconductor package including attaching a capacitor to a substrate. In a first step 12, solder paste is dispensed onto specific locations on a substrate and at step 14, a capacitor is attached to the substrate using the solder paste. A flux is dispensed onto controlled collapse chip connection (C4) pads on the substrate at step 16, and a bumped integrated circuit (IC) die is placed on the fluxed C4 pads at step 18. A reflow operation is performed at step 20. The reflow operation melts the solder paste, forming solder joints between the capacitor and the substrate. At step 22, an underfill is dispensed into gaps between the substrate and the bumped IC die, thereby forming the packaged semiconductor device.
However, a number of problems are encountered in current solder attachment processes used to mount capacitors onto substrate surfaces. These include solder balling, tombstoning and excess solder. Each of these problems is a cause for rejecting a semiconductor unit.
Solder balling is a soldering defect characterised by the presence of tiny solder balls trapped along a peripheral edge of the solder joint. An example of a solder balling defect is shown in FIG. 2. In FIG. 2, a capacitor 30 having a first end termination 32 and a second end termination 34 is shown. The first and second end terminations 32 and 34 of the capacitor 30 are respectively attached to a first capacitor pad 36 and a second capacitor pad 38 on a substrate 40 via respective first and second solder joints 42 and 44. As can be seen, a solder ball 46 is formed on a surface of the first solder joint 42. The presence of a number of such solder balls 46 often leads to capacitor shorting.
Tomb stoning is a phenomenon where one end of a component is lifted off a substrate, while the opposite end of the component remains bonded to the substrate. An example of a tombstoning defect is shown in FIG. 3. FIG. 3 shows a capacitor 50 having a first end termination 52 and a second end termination 54. The first end termination 52 of the capacitor 50 is bonded to a first capacitor pad 56 on a substrate 58 via a joint 60 formed of a solder material 62. As can be seen, the second end termination 54 of the capacitor 50 is raised and detached from a second capacitor pad 64 on the substrate 58. The tomb stoning defect shown in FIG. 3 may be caused by unequal volumes of solder material 62 deposited on the first and second capacitor pads 56 and 64.
Referring now to FIG. 4, an example of excess solder will now be described. FIG. 4 shows a capacitor 70 having a first end termination 72 and a second end termination 74. The first and second end terminations 72 and 74 of the capacitor 70 are respectively attached to a first capacitor pad 76 and a second capacitor pad 78 on a substrate 80 via respective first and second solder joints 82 and 84. However, instead of having desirable concave profiles 86 and 88 (defined by dashed lines in FIG. 4), the first and second solder joints 82 and 84 have convex profiles 90 and 92 (defined by solid lines in FIG. 4) due to the presence of excess solder. Excess solder is undesirable as too much solder can result in a bridging defect, which will cause an electrical short.
Besides losses from rejecting semiconductor units with solder attachment problems, material, labour and machine costs are also incurred when solder paste is used in the capacitor attachment process. All these add to the overall manufacturing cost.
In view of the foregoing, it would be desirable to have a method of attaching a capacitor to a substrate without using solder paste.